We present an interferometric technique based on differential interferometry setup for measurement in the subnanometer
scale in atmospheric conditions. One of the important limiting factors in any optical measurement are fluctuations of the
refractive index of air representing a source of uncertainty traditionally compensated when the index is evaluated
indirectly from the physical parameters of the atmosphere. Our proposal is based on the concept of overdetermined
interferometric setup where a reference length is derived from a mechanical frame made from a material with very low
thermal coefficient on the 1*E-8 level. The technique allows to track the variations of the refractive index of air on-line
directly in the line of the measuring beam and to compensate for the fluctuations. The optical setup consists of three
interferometers sharing the same beam path where two measure differentially the displacement while the third evaluates the changes in the measuring range acting as a tracking refractometer. The principle is demonstrated on an experimental setup and a set of measurements describing the performance is presented.